Background: Quantitatively assessing host immunity remains a challenge in clinical practice. Results: Most parameters in lymphocyte number, function and phenotype were correlated with age. The reference ranges of these parameters were established in four age groups (children, adolescents, adults, and elders). The numbers of CD4(+) T cells, CD8(+) T cells, B cells, but not NK cells, were negatively correlated with age. However, the function of CD4(+) T cells, CD8(+) T cells and NK cells was positively correlated with age. The expression of CD28 on T cells gradually decreased with increasing age and was negatively correlated with their function. An opposite phenomenon was observed in the expressions of HLA-DR and CD45RO on T cells. An immune scoring model was established by using 8 parameters (CD4(+) T cell number x function, CD28(+)CD4(+) T cell number, HLA-DR(+)CD4(+) T cell number, CD45RO(+)CD4(+) T cell number, CD8(+) T cell number x function, CD28(+)CD8(+) T cell number, HLA-DR(+)CD8(+) T cell number, NK cell number x function) from the results of lymphocyte number, function, and phenotype. This immune scoring model showed sensitivities of 70% and 71.4% in determining hyper-immune and hypo-immune status, respectively. Conclusions: An immune scoring model based on combination of lymphocyte number, function, and phenotype shows potential value in quantitatively assessing host immunity. Methods: 261 healthy individuals aged 1 to 82 years were recruited from Tongji Hospital. The number, function, and phenotype of CD4(+) T cells, CD8(+) T cells and NK cells were simultaneously determined.